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This invention relates to filters used in power electronic devices such as switch-mode power supplies. More specifically, the invention relates to integrated-magnetic filters that provide both common-mode and differential mode inductance.
Power electronic devices generate radio frequency noise that can be conducted to output leads or back through power lines linked thereto. This noise can interfere with operation of other electronic equipment linked to the power lines or the outputs. In addition, normal operation of power electronic devices can be disturbed by noise and transients present on power supply lines. It is therefore desirable to place a filter at the input of these devices in order to provide a level of isolation between the devices and the power system.
Noise currents can be described in terms of differential-mode and common-mode noise components. Differential-mode noise components consist of currents of equal magnitude flowing in opposite directions in supply and return lines. Common-mode noise components consist of currents of equal magnitude flowing in the same direction in both the supply and return lines. The return path for common-mode currents is a ground connection.
Differential-mode noise is typically filtered by placing one or more inductors in series with the supply line, the return line or both. Common-mode noise is usually filtered by placing a pair of coupled inductors wound on the same core in series with the supply and return lines. In order to save space and reduce cost, integrated-magnetic filters which provide both common-mode and differential-mode noise attenuation have been devised.
U.S. Pat. No. 5,155,676 (the ""676 patent) teaches several different general structures that can be used to configure common mode-differential mode inductors. In particular, the ""676 patent teaches one structure including a xe2x80x9cFigure 8xe2x80x9d core comprising three legs that traverse the distance between top and bottom core members. The legs and members define two windows in the core. The ""676 patent teaches that two identical windings can be provided, one on each outer leg where each winding begins at a supply side, passes from front to back through a respective window and wraps around the corresponding leg several times before ending at a load end. The windings form identical turns about the legs.
In this case, common mode noise currents in each winding pass through the respective core windows going from front to back of the core. The associated magnetic fluxes in the core add in a flux path through the outer legs, and subtract in the center leg. The net common-mode flux thus encircles both windows, with no flux in the center leg.
Differential mode noise currents, on the other hand, pass through corresponding core windows going in different directions with respect to the front and back of the core. For instance, if the differential mode current in a first winding goes from front to back, the current in the second winding passes through the other core window from back to front. Magnetic fluxes produced by the differential mode currents travel in opposite directions in the outer legs so that net differential-mode flux encircles each window, with twice the flux in the center leg as in each outer leg.
Such integrated-magnetic assemblies are often used to filter unwanted high frequency noise on conductors which carry DC power to electronic devices or equipment. Thus, these integrated-magnetic assemblies must provide filtering for common and differential noise while accommodating the differential current delivering power. In general, the larger the inductance for each mode, the larger the attenuation provided for the noise. The desire is then to increase both the differential and common-mode inductance in the integrated-magnetic assembly in order to provide increased noise attenuation. However, since the differential flux path must accommodate flux associated with power flow, while the common-mode need not, the design considerations for the two inductances are different.
The common-mode inductance in the ""676 structure described above is obtained using a flux path around both windows. The inductance associated with this path increases directly with increasing permeability of the core material in this path. For a given material, permeability is maximized if there are no air gaps in the path. Therefore, for increased noise attenuation, a common-mode flux path will be formed of high-permeability material arranged to form an un-gapped flux path.
The differential-mode inductance in an integrated-magnetic assembly like the one described in the ""676 patent is obtained using the flux path through the center leg. This magnetic flux path must accommodate the flux from the AC line or DC power without exceeding the saturation flux density of any material in the flux path. A standard practice in the design of a differential inductor is to introduce an air gap to limit the flux. An air gap increases the reluctance of the flux path (i.e., decreases the ease with which flux flows in the path). This increased reluctance allows less flux to flow for a given current in the windings, and thus helps to keep the flux density level below the saturation level of the materials, but reduces the inductance, compared to an un-gapped path. The ""676 patent teaches one structure where the center leg forms a gap as described herein.
As well known in the art, one way to increase permeability is to form the core from a plurality of laminates. To this end, one core configuration that has particularly advantageous operating characteristics includes flat steel laminates stacked to form a Figure 8 core configuration with a gap in the center leg as described above. While this solution has advantageous operating characteristics, the laminate stacking task can be labor intensive and therefore relatively expensive.
It has been recognized that all of the benefits of prior art common mode/differential mode inductance assemblies can be accomplished with a new assembly configuration that, at lest in some respects, is less expensive to produce than other configurations that operate efficiently. To this end, an exemplary embodiment of the invention includes an assembly having improved damping characteristics and comprising a first torridial core of magnetic material having at least first and second core segments and forming a first surface, a fist winding encircling the first core segment, a second winding encircling the second core segment, the first and second winding segments wrapped in the same direction about the core segments and a second torridial core forming a second surface and secured proximate the first torridial core and outside the windings such that the first and second surfaces oppose each other and form an air gap there between.
In one aspect the first torridial core provides a common mode inductance and the first and second cores and the gap provide a damped differential mode inductance.
In several embodiments the torridial cores are secured together by an epoxy. In some designs the epoxy fills the space within the first torridial core and may also fill the space within the second torridial core. The epoxy may form at least one mounting aperture within the first torridial core. The assembly may further including an insulative spacer between the first and second cores.
Thus, the epoxy can perform several functions including electrically insulating the first and second cores, providing the gap between the two cores and providing a mounting mechanism for the assembly.
Either or both of the first and second cores may include a ribbon of metallic material wrapped in torrid form. The first torrid in some embodiments is coated with an insulating varnish binder material.
In this regard, it has been recognized that the labor involved with wrapping a metallic ribbon to form a torrid is substantially less than that required to stack laminates to form a Figure 8 core or some of the other cores that have been attempted in the industry. Thus, the present invention should reduce manufacturing costs while still providing an assembly that performs effectively).
The first torridial core may be formed about an axis and the first surface may be perpendicular to the axis. In the alternative, the first surface may be an internal surface of the first torridial core and the second torridial core may be positioned within the first torridial core.
The invention also includes a method for forming an electromagnetic assembly having improved damping characteristics, the method comprising the steps of providing a first torridial core of magnetic material having at least first and second core segments, the first torridial core forming a first surface, winding a first winding about the first core segment, winding a second winding about the second core segment and providing a second torridial core adjacent the first torridial core such that a second surface formed by the second core opposes the first surface and the first and second surfaces form a gap there between.
The step of providing the second torridial core may include providing the second torridial core adjacent the first torridial core. In the alternative, the step of providing the second torridial core may include providing the second core within the first torridial core.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.